iwlwifi: mvm: use mvmsta consistently in rs.c

We use mvmsta for the sta->drv_priv in mvm, but in rs.c we have a
bunch of instances using sta_priv, which is probably due to it being
copied from dvm.  Change all occurrences to mvmsta for consistency
with the rest of the driver

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>